Method for firing a rotary kiln with pulverized solid fuel

Abstract

Disclosed is a method for firing a kiln as well as a method for producing cement clinker in which pulverized coal is initially entrained in an airflow of about 2% of the theoretical amount of air needed to combust the coal and transport it to a burner. Supplemental primary air heated sufficiently to vaporize volatiles in the coal is mixed with the coal flow in a burner, discharged into the kiln and hence ignited. Secondary combustion air heated to between 800° F. to 1650° F. and more is added in the kiln to effect the substantially complete combustion of the pulverized coal in the kiln.

We claim: 
     
       1. A method for heating a rotary kiln having an open end from which product being treated in the kiln is discharged by injecting through the end into the kiln and combusting in the kiln pulverized coal including volatiles at a rate so that a desired temperature is maintained within the kiln, the method comprising the steps of: providing a burner including a flame tube extending into the kiln;   generating a first, pressurized primary airflow comprising no more than about 2% of the theoretical amount of air needed to combust the coal in the kiln;   entraining in the first primary airflow the pulverized coal at said rate to form a pulverized coal flow;   directing the pulverized coal flow into the tube;   adding a second, supplemental primary airflow having a temperature of at least about 600° F. to the pulverized coal flow in the tube to form an air-pulverized coal stream;   regulating the flow rate of the second primary airflow so that the first and second airflows comprise no more than about 15% of the theoretical amount of air needed to combust the coal in the kiln;   retaining the stream in the tube for a sufficient length of time to evaporate volatiles present in the coal while the stream is in the tube;   thereafter discharging the stream including evaporated volatiles into the kiln;   igniting the stream;   generating a secondary combustion airstream having a temperature of at least about 800° F.; and   adding a sufficient amount of secondary air to the stream injected into the kiln to effect a substantially complete combustion of the pulverized coal in the kiln.   
     
     
       2. A method according to claim 1 wherein the step of regulating the flow rate of the second, supplemental primary airstream comprises the step of limiting the amount of air therein to no more than about 8% of the theoretical amount of combustion air. 
     
     
       3. A method according to claim 2 including the step of adding up to about 25% by weight of pulverized petroleum coke to the pulverized coal. 
     
     
       4. A method for producing clinker for the subsequent conversion thereof into the cement, the method comprising the steps of: providing an elongate, longitudinally inclined tubular kiln having a relatively higher intake end and a relatively lower discharge end;   introducing clinker raw materials through the intake into the kiln;   rotating the kiln so that the materials travel towards the discharge end;   providing an elongate burner tube at the discharge end which is oriented substantially parallel to the kiln, and which has an inner end extending into the kiln and an outer end;   flowing a first, pressurized primary airstream into the outer end of the tube;   forming a second flow of supplemental primary air having a temperature of at least about 600° F.;   combining the first and second airflows in the vicinity of the tube and directing the combined airflow through the tube into the kiln;   entraining pulverized coal in the first airflow at a rate selected so that the complete combustion of the coal in the kiln heats the materials in the kiln to a desired temperature;   limiting the flow rate of the first airflow so that the second airflow provides a majority of the combined air flowing through the tube and further so that the combined air comprises not more than about 15% of the theoretical amount of air required to combust the pulverized coal in the kiln;   retaining the combined first and second airflows and the entrained coal in the tube for a sufficient length of time so that the supplemental primary air heats the pulverized coal in the tube sufficiently to vaporize volatiles present in the coal;   thereafter discharging the pulverized coal from the inner end of the burner tube; and   directing a secondary combustion airflow through the discharge end in substantially full surrounding relationship to the tube into the kiln, the secondary combustion airflow having an amount of air sufficient to fully combust the pulverized coal discharged from the inner tube end into the kiln and further having a temperature substantially in excess of the temperature of the second airflow.   
     
     
       5. A method according to claim 4 including the step of limiting the combined airflow to a flow rate so that it provides no more than about 10% of the theoretical amount of combustion air. 
     
     
       6. A method according to claim 5 including the step of limiting the flow rate of the first airflow so that it provides no more than about 2% of the theoretical amount of air. 
     
     
       7. A method according to claim 4 wherein the burner tube is surrounded by a plurality of fluid fuel burners equally and substantially concentrically distributed about the burner tube, and including the step of periodically activating the fluid fuel burners to at least partially heat the materials in the kiln with the fluid fuel burners.